L0301P57 - Molecular Basis of Cancer 2
Cancer Regulating Genes *several types of proteins play a key role in regulating cell growth and have a significant role in the development of cancer *identification of these genes have come from studies in which cancer cells have been examined and the genes which are altered are identified Genes that: *promote cancer - oncogenes *act to prevent cancer - tumour suppressor *repair mutated DNA - mismatch repair Proto-Oncogenes *normal cellular genes that promote cell growth and cell division *extremely important for normal cell survival, growth, development and differentiation *falls into five major classes of gene: **growth factors (GFs) **growth factor receptors **intracellular signalling proteins **transcription factors **cell cycle control proteins Proteins encoded by proto-oncogenes: *GF Receptors **oncogenic mutation in these in every human cancer *Kinases **intracellular signalling proteins that phosphorylates/activates something else *Cyclins and CDKs **fault in regulation of the cell cycle *Apoptosis Regulating Proteins **results in cells that will not die *Transcription Factors **increased gene expression in genes involved in cell growth Oncogenes *altered forms of proto-oncogenes that lead to increased activity of gene products and can cause cancer *any class of porto-oncogene can be mutated to become an oncogene **mutation may be in only one copy of the gene but must result in increased activity for cancer predisposition to occur *mutant form are overactive and signal cells to divide inappropriately and promotes cell growth in inappropriate circumstances *over 100 identified *encountered repeatedly in many cancers, frequently leukemias *promote cell proliferation - active inappropriately in cancer *three main ways a proto-oncogene can become an oncogene: **activating mutation in coding sequence **gene amplification **chromosomal translocation *act in a dominant matter **a gain-of-function mutation in a single copy of a cancer-critical gene can predispose to cancer Intracellular Signalling Cascades *during embryonic development and adult life growth promoting signals need to be precisely regulated *cell signalling needs to be integrated with the body’s needs *certain classes of proteins - signalling proteins are more often mutated in human cancers Tyrosine Kinases *an important group of signalling proteins frequently mutated in cancers to become more active (oncogenic) *initiate an intracellular cascade of signals that regulate cellular proliferation, survival, differentiation, function, and motility *major mechanism of cancer development and represent new therapeutic targets for the treatment of cancer *constitutive activation of tyrosine kinases **means TKS are permanently activated **may occur by several mechanisms **is associated with every type of human cancer Tyrosine Kinase Receptors *largest group of oncogenes *transmembrane spanning receptors that transduce the signals of extracellular GFs **extracellular - growth factor binding site **intracellular - kinase domain *intrinsic enzyme activity in cytoplasmic domain - stimulated by ligand (GF) binding *normally, receptor tyrosine kinase activity in the resting cell is tightly controlled **encode receptors for growth factors or cytokines and when stimulated result in cell division and proliferation **signal through the Ras/Map kinase pathway to activate transcription factors *when mutated or over-expressed these receptors can cause cellular transformation (cancer) – oncogenes Receptor Activation *receptors are activated by ligand binding, usually growth factors *binding of the ligand (eg EGF) to the receptor causes the receptors to cluster and assemble into dimers which enables two receptors to cross phosphorylate each other *autophosphorylation occurs to initiate an intracellular signalling cascade **once activated, receptors transfer a phosphate group from ATP to selected tyrosine side chains, both on the receptor itself or on other cellular proteins **leads to the recruitment of other proteins to enable binding to the receptor *autophosphorylated tyrosine on the receptor serves as a binding site for a number of intracellular proteins *each of these proteins binds to a different phosphorylated site *once bound to the receptor, many of the signalling proteins are themselves phosphorylated and thereby activated *∴ tyrosine phosphorylation is a switch to trigger assembly of signalling complexes and relays signals to the nucleus   Kinase Mutations in Cancer *receptor tyrosine kinases are mutated in breast, lung, brain, prostate and renal cancer causing more active kinase activity *mutated src non-receptor tyrosine kinases (do not have extracellular binding site) are more active in bowel cancer *mutated Akt and PDK serine threonine kinases are more active in breast and ovarian cancers  E.g.: Chronic Myeloid Leukaemia (CML) *one of rare cancers which will occur due to the effect of just one oncogene *was fatal (3-5 yrs) in 80’s and 90’s however is now curable with new therapies *one of most common leukemias **20% of leukemias and often affects patients in their 30’s and 40’s   *originates from a single abnormal haematopoietic stem cell *all leukaemic cells demonstrate translation in chromosomes 9 and 22 *characterised by proliferation of immature white blood cells in bone marrow and peripheral circulation *CML cells proliferate unchecked leads to: **bone marrow replacement **increase in liver and spleen size **large increase in white cells in circulation *diagnosis requires showing the chromosomal translocation by standard cytogenetics (karyotyping), fluorescent in-situ hybridisation (FISH) or PCR Abl Tyrosine Kinase *located on chromosome 9 *plays a major role in regulating cell cycle progression (normally tightly regulated) **when over-expressed it actually inhibits cell cycle progression *when fused to breakpoint cluster region (BCR) on chromosome 22, becomes overactive causing increased cell growth and inhibition of cell death CML Chromosome Translocation *translocation forms Philadelphia chromosome = shortened chromosome 22 **reciprocal translocation of abl TK from long arm of chromosome 9 to BCR of long arm of chromosome 22 *results in generation of fusion protein between BCR and abl TK which has increased TK compared to normal abl *BCR-abl fusion protein causes leukaemia STI-Gleevec (Imatinib Mesylate) *most cancer drugs found by random screening of chemicals to see if they kill cancer cells *STI-571 (signal transduction inhibitor) designed to inhibit activity of abl TK **prevents binding of ATP to TK **now in clinical treatment for CML and is showing great efficacy Cancer Treatment Chemotherapy *aims to kill most rapidly dividing cells *will kill cancer cells plus normal cells that divide rapidly including bone marrow, gut etc *∴ high dose chemotherapy associated with significant side effects Surgery *cut the cancer out but not always possible Targeted Therapies *find out what the molecular changes in the cancer are and design therapies that target the molecular rearrangement *targeted approach is complementing chemotherapy and giving new hopes for cures of incurable cancers and new options for treatments of resistant cancers   Pharmacological Inhibition of TKs ATP-competitive inhibitors: *idea behind anti-tyrosine kinase drug discovery is to find small molecules that directly inhibit the catalytic activity of the kinase by interfering with the binding of ATP or substrates – e.g. STI-571 (as above)  'Other anti-tyrosine kinase drugs: ' *antibodies against receptor tyrosine kinases or their ligands *interrupt kinase activation signaling through neutralization of ligand   'Antibodies against receptor TKs or ligands ' *blockade of ligand binding, receptor internalisation, and perhaps antibody-mediated cytotoxicity *e.g.: breast cancer - EGFR - Her-2 'Anti-Angiogenics ' *preventing angiogenesis, and thus inhibit cancer growth and proliferation e.g. VEGF Novel Cancel Treatments - TK Inhibition *TKs have emerged as clinically useful drug targets for treating gastrointestinal cancer *STI-571 was primarily designed to treat CML but is also an inhibitor of c-kit receptor tyrosine kinase **currently the drug of choice for the therapy of metastatic gastrointestinal stromal tumors (GISTs), which frequently express constitutively activated forms of the c-kit receptor